Tunable diode laser spectroscopic analysis of gaseous samples is known, see, for example, U.S. Pat. No. 6,615,142. The method of the '142 patent relies on a computation with the assumption that the sample spectrum can be fitted as a linear combination of its pure constituents. However, when the concentration of the components of interest is relatively low in the sample matrix, then, for example, the computation of the '142 patent fails to accurately determine the concentration of the components of interest. For example, when the sample matrix is the gaseous stream from an ethylene cracker acetylene hydrogenator and the component to be analyzed in the sample is acetylene at a concentration of about 1 part per million, then the computation of the '142 patent fails to accurately estimate the concentration of the relatively low concentration of acetylene. It would be an advance in the art if a tunable diode laser based spectroscopic method could be discovered that solved the above-stated problems of the prior art.
The determination of such low levels of acetylene is important for ethylene cracker hydrogenators. An ethylene cracker produces undesirable acetylene along with the desired ethylene and propylene. The purification system does not remove the undesired acetylene. Therefore, a hydrogenator is used to hydrogenate the acetylene to ethylene. Ethylene cracker hydrogenators are generally of two types. The “front end” hydrogenator is fed directly from the cracker and contains hydrogen as made in the cracker. The “back end” hydrogenator is usually fed from a purification system which receives its feed from the cracker and must add hydrogen to the reactor because it was removed in a prior purification step. The hydrogenator is fed a controllably heated gaseous stream from the cracker or from the purifier.
The heart of an ethylene cracker hydrogenator is a bed of catalytic material which catalytically reacts the acetylene with the hydrogen. In the art, filter photometry is typically used to determine acetylene in the range of from, for example, 0.3 to 1% in the feed stream while gas chromatography is typically used to determine acetylene in the range of, for example, about 1 part per million in the outlet stream from the hydrogenator. There is little dissatisfaction with the filter photometry analysis of the inlet stream for acetylene. However, gas chromatography does not provide sufficiently rapid analysis of the outlet stream for acetylene to prevent, for example, off-spec product or a thermal run-away of the hydrogenator system. It would be an advance in the art if a method could be discovered that solved the above-stated problems of the prior art ethylene cracker hydrogenator control systems.